Introduction

Who never wondered how virtual machines work? But, better than creating your own virtual machine, what if you can use one done by a big company focusing on improving the speed to the VM? In this article, I'll introduce how to use the V8 in your application, which is Google's open source JavaScript engine inside Chrome - Google’s new browser.

Background

This code uses the V8 as an embedded library to execute JavaScript code. To get the source of the library and more information, go to the V8 developer page. To effectively use the V8 library, you need to know C/C++ and JavaScript.

Using the Code

Let's see what is inside the demo. The demo shows:

How to use the V8 library API to execute JavaScript source code.

How to access an integer and a string inside the script.

How to create your own function which can be called inside the script.

How to create your own C++ class which can be called inside the script.

First, let's understand how to initialize the API. Look at this simple example of embedded V8 on C++:

Well, but this doesn't explain how we can control variables and functions inside the script. OK, the script does something... but, we need to get this information somehow, and have customized functions to control specific behavior inside the script.

The Global Template

First, we need a global template to control our modifications:

v8::Handle<v8::ObjectTemplate> global = v8::ObjectTemplate::New();

This creates a new global template which manages our context and our customizations. This is important because in V8, each context is separated, and can have its own global template. In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8.

Customized functions need to receive const v8::Arguments& as a parameter and must return v8::Handle<v8::Value>. Using the global template pointer created before, we add the function to the template, and associates the name "plus" to the callback "Plus". Now, each time we use the "plus" function on our script, the function "Plus" will be called. The Plus function does just one thing: get the first and the second parameters and return the sum.

OK, now we can use a customized function in the JavaScript side:

plus(120,44);

and we can use this function's result inside the script:

x = plus(1,2);
if( x == 3){
// do something important here!
}

Accessors - Variables Accessible Inside the Script!

Now, we can create functions... but wouldn't it be cooler if we can use something defined outside inside the script? Let's do it! The V8 have a thing called Accessor, and with it, we can associate a variable with a name and two Get / Set functions, which will be used by the V8 to access the variable when running the script.

global->SetAccessor(v8::String::New("x"), XGetter, XSetter);

This associates the name "x" with the "XGetter" and "XSetter" functions. The "XGetter" will be called by V8 each time the script needs the value of the "x" variable, and the "XSetter" will be called each time the script must update the value of "x". And now, the functions:

In the XGetter, we only need to convert "x" to a number type value managed by V8. In the XSetter, we need to convert the value passed as parameter to an integer value. There is one function to each basic type (NumberValue for double, BooleanValue for bool, etc.)

For the string, things changed a little. The "userGetter" creates a new V8 string in the same way the XGetter does, but the userSetter needs first to access the internal string buffer by using the ToString function. Then, using the pointer to the internal string object, we use the WriteAscii function to write its content to our buffer. Now, just add the accessor, and voila!

Printing

The "print" function is another customized function which catches all parameters and prints them using the "printf" function. As we have done before with the "plus" function, we register our new function on the global template:

//associates print on script to the Print function
global->Set(v8::String::New("print"), v8::FunctionTemplate::New(Print));

For each parameter, the v8::String::AsciiValue object is constructed to create a char* representation of the passed value. With it, we can convert other types to their string representation and print them!

Sample JavaScript

Inside the demo program, we have this sample JavaScript to use what we have created so far:

Well, GetFunction returns the point constructor (on the JavaScript "side"), and with it, we can create a new instance using NewInstance. Then, we set our internal field (which we have created "space" for with SetInternalFieldCount) with the class pointer. With that, the JavaScript will have access to the object through the pointer.

Only one step is missing. We only have a class template and instance, but no name to access it from the JavaScript:

context->Global()->Set(String::New("point"), obj);

This last step links the "point" name to the instance obj. With these steps, we just emulate the creation of class Point with the name point in our script (before loading or running it!).

Accessing a Class Method Inside the JavaScript

OK, but this doesn't explain how we can access Function_A inside the Point class...

Like a normal accessor, we must handle the arguments. But, to have access to our class, we must get the class pointer from the internal field (first one). After mapping the internal field to "wrap", we get the class pointer by retrieving its "value". Now, it's a matter of cast...

Now, we have the class pointer for the "point" class being used (which called method_a, which called the callback PointMethod_A).

I hope this helps, and if something is wrong or unclear, please don't hesitate to contact.

Points of Interest

Before trying to use the V8 library, I've tried to use Lua as an embedded scripting language to support easy configuration and changes by the user. I got amazed by V8 as it's faster and easier to use than Lua to implement scripting functionality in my own software. I hope this article help others in this task.

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About the Author

I did extremly different works: services to control harware, internet banking sites, Operational System migration (from Digital to Aix , from HpUX to Linux and from tru64 4.0 to 5.1b), Grafical user interfaces for lots of programs and different OS's...
I also know and had use Delphi, c, c++, java, python, assembly and perl.

Hi,
sorry but I've tried to fix this quot; stuff everytime I update the article. It seems that the CodeProject wizard is having trouble with my post. I tried to use internet explorer, firefox and chrome and in all of them the source end with this ugly quot; istead of ".
Hope this will be fixed soon